In recent years, it has been required to communicate a large capacity of data at high speed in a radio communication, and demand for transmitting a multi-valued digitally-modulated signal rises accordingly. Generally speaking, transmission information is superimposed not only on a phase component but also on an amplitude component in the multi-valued digitally-modulated signal. Due to this, a radio transmitter apparatus transmitting the multi-valued digitally-modulated signal is required to have a quite high linearity. On the other hand, as portable radio communication apparatuses typified by a mobile telephone have become popular, the radio transmitter apparatus is required to realize low power consumption, that is, a transmission power amplifier apparatus is required to operate with high efficiency. Generally speaking, the linearity and the highly efficient operation of the transmission power amplifier apparatus hold a trade-off relationship. If electric power is amplified while giving greater importance to the linearity, the power added efficiency tends to be sacrificed. If electric power is amplified while giving greater importance to the power added efficiency, the linearity tends to be sacrificed.
Various methods have been conventionally proposed to satisfy both the linearity and the highly efficient operation for the power amplifier. As one of power amplifying methods according to a prior art, there has been known a method of linear amplification using nonlinear components (referred to as a LINC method hereinafter). A LINC amplifier performs a vector resolution to divide a modulated signal an amplitude component of which temporally changes into two constant amplitude signals with an arbitrary phase difference between them, amplifies each of the constant amplitude signals using a nonlinear amplifier capable of performing highly efficient amplification, and then performs a vector combination, and this leads to satisfying both the linearity and the high efficiency. The LINC is advocated by D. C. Cox and disclosed in D. C. Cox, “Linear amplification with nonlinear components”, IEEE transactions on communications, December 1974, COM-22, pp. 1942-1945. Furthermore, an applied LINC amplifier is disclosed in Japanese Examined Patent Publication No. 6-22302, Japanese Patent No. 2758682, and U.S. Pat. No. 5,287,069.
FIG. 23 is a block diagram showing a configuration of a LINC amplifier 10 according to a prior art. FIG. 24A shows an example of a vector combination when an amplitude value of a combined output signal of the LINC amplifier 10 shown in FIG. 23 is large. FIG. 24B shows an example of the vector combination when the amplitude value of the combined output signal of the LINC amplifier 10 shown in FIG. 23 is small. Referring to FIG. 23, the LINC amplifier 10 is configured to include a signal calculating unit 21 that performs a vector resolution to divide a modulated signal having a temporally-changing amplitude value into two constant amplitude signals, power amplifiers 31 and 32 that amplify the two constant amplitude signals, respectively, and a power combiner 41 that combines signals outputted from the power amplifiers 31 and 32, generates a combined output signal, and outputs the combined output signal. In this case, when a modulated signal S(t) inputted to the LINC amplifier 10 is represented by the following Equation (1), constant amplitude signals S1(t) and S2(t) outputted from the signal calculating unit 21 to the power amplifiers 31 and 32 are represented by the following Equations (2) and (3) as shown in FIGS. 24A and 24B, respectively:
                                          S            ⁡                          (              t              )                                =                                    A              ⁡                              (                t                )                                      ⁢                          cos              ⁡                              (                                                                            ω                      c                                        ⁢                    t                                    +                                      ϕ                    ⁡                                          (                      t                      )                                                                      )                                                    ,                            (        1        )                                                      S            ⁢                                                  ⁢            1            ⁢                          (              t              )                                =                                                    A                ⁢                                                                  ⁢                max                            2                        ⁢                          cos              ⁡                              (                                                                            ω                      c                                        ⁢                    t                                    +                                      ϕ                    ⁡                                          (                      t                      )                                                        +                                      θ                    ⁡                                          (                      t                      )                                                                      )                                                    ,                                  ⁢        and                            (        2        )                                                      S            ⁢                                                  ⁢            2            ⁢                          (              t              )                                =                                                    A                ⁢                                                                  ⁢                max                            2                        ⁢                          cos              ⁡                              (                                                                            ω                      c                                        ⁢                    t                                    +                                      ϕ                    ⁡                                          (                      t                      )                                                        -                                      θ                    ⁡                                          (                      t                      )                                                                      )                                                    ,                            (        3        )            
where the phase θ(t) is represented by the following Equation (4):
                                          θ            ⁡                          (              t              )                                =                                    cos                              -                1                                      ⁡                          (                                                A                  ⁡                                      (                    t                    )                                                                    A                  ⁢                                                                          ⁢                  max                                            )                                      ,                            (        4        )            
where A(t) denotes an amplitude of the modulated signal S(t), ωc denotes a carrier frequency of the modulated signal S(t), φ(t) denotes a phase of the modulated signal S(t), Amax denotes a maximum value of the amplitude of the modulated signal S(t), and θ(t) denotes a phase difference between each of the constant amplitude signals S1(t) and S2(t) and the modulated signal S(t).
As can be seen from above, the modulated signal S(t) is divided into the two constant amplitude signals S1(t) and S2(t) each having a constant amplitude Amax/2. As the power amplifiers 31 and 32, nonlinear amplifiers capable of performing highly efficient operation can be employed. Therefore, the LINC amplifier 10 according to the prior art can realize a linear amplification operation as a whole of the LINC amplifier 10, using the nonlinear amplifiers capable of performing highly efficient operation.
However, the LINC amplifier 10 according to the prior art has the following problems if, for example, the inputted modulated signal S(t) is a multi-valued digitally-modulated signal in which transmission information is superimposed on an amplitude component, and a dynamic range indicating a change width of an amplitude is wide, or when the combined output signal is obtained in a wide range from a low output to high output, e.g., a peak to average ratio (referred to as a PAR hereinafter) is high, in particular, when the low output is obtained. As shown in FIG. 24B, when the amplitude of a combined output signal E(t) after electric power combination is small, that is, the amplitude of the modulated signal S(t) is small, then the phase difference is greater between the two constant amplitude signals S1(t) and S2(t), reactive power at the time of power combination by the power combiner 41 increases, and combined loss increases. Therefore, even if the power amplifiers 31 and 32 perform highly efficient nonlinear amplification on the respective constant amplitude signals S1(t) and S2(t), the overall efficiency of the LINC amplifier 10 is not so improved.